KR102493979B1 - High-strength steel plate for pressure vessels with excellent impact toughness and manufacturing method thereof - Google Patents

Abstract

A high-strength steel for pressure vessels having excellent impact toughness and a manufacturing method thereof are provided.
The high-strength steel for pressure vessels with excellent impact toughness of the present invention, in weight percent, contains carbon (C): 0.04-0.08%, manganese (Mn): 1.0-2.0%, silicon 0.01-0.5%, phosphorus (P): 0.02 % or less, Sulfur (S): 0.01% or less, Chromium (Cr): 1.0 to 2.0%, Molybdenum (Mo): 0.2 to 0.8%, Aluminum (Al): 0.005 to 0.5%, Titanium (Ti) 0.01 to 0.03% , Nitrogen (N): 0.001-0.006, the balance including Fe and other unavoidable impurities, in area%, 70-90% self-tempered martensite, and the balance self-tempered bainite steel microstructure. have

Description

High-strength steel for pressure vessels with excellent impact toughness and its manufacturing method

The present invention relates to the manufacture of steel materials used as materials for plants, pressure vessels, storage tanks, etc., and more particularly, to high-strength steel materials for pressure vessels with excellent impact toughness and a manufacturing method thereof.

Steel materials used as materials for pressure vessels, plants, storage tanks, etc. are usually manufactured by performing heat treatment such as normalizing, quenching-tempering, etc. in order to secure target quality such as strength and impact toughness. High strength and low-temperature impact toughness, along with thick thickness, are the main characteristics required for steel for pressure vessels in recent years, according to the trend of increasing the size of pressure vessels and the harshness of the use environment. However, among the mechanical properties of steel, strength and low-temperature impact toughness generally tend to be inversely proportional, so the application of more advanced technology is required to secure low-temperature impact toughness in high-strength steel.

In general, as the temperature of use of steel decreases, the toughness decreases, which adversely affects stability when used at low temperatures. In particular, as the thickness increases and the closer you get to the center of the steel, the perceived cooling rate decreases, so the opportunity for grain size to grow relatively increases, and as a result, the strength and impact toughness of the internal structure, which is greatly affected by the grain size, increases to a greater extent. shows a declining trend. Therefore, the composition or microstructure of a steel having a low service temperature must be controlled so that strength or low-temperature impact toughness do not deteriorate.

When high strength is required among steel products manufactured by performing heat treatment, it is mainly manufactured through a quenching-tempering heat treatment process. After hot rolling, the steel material cooled to room temperature is heated and maintained at a temperature of Ae ₃ or higher to create austenite, and then, through rapid cooling, the phase transformation of the low-temperature structure including martensite or bainite is promoted to secure strength (?? ). In addition, while the structure generated through rapid cooling has high strength, internal stress causes poor ductility and toughness, so even a small impact in the toll processing and use environment of steel can cause fracture. To compensate for this, stress at temperatures below Ae ₁ For the purpose of mitigation, heat treatment is performed to ensure the quality of the final steel (tempering). At this time, the tensile strength of the steel material is slightly reduced, but the effect of greatly improving ductility and toughness can be expected. In quenching-tempering heat treatment applied steel material, the mechanical properties are greatly affected by the tempering heat treatment temperature and time. Quenching-tempering heat treatment is the most common method to secure ductility and impact toughness of high-strength steel materials, but high alloys are used to secure high hardenability compared to other steel plate manufacturing methods such as thermal processing control process (TMCP) or normalizing heat treatment. There is a disadvantage in that the process cost increases because the addition amount of elements is required and two types of heat treatment processes, quenching and tempering, are required.

On the other hand, the alloy components of the steel material affect the mechanical properties of the steel material applied with quenching-tempering heat treatment in two aspects. First, it affects the hardenability of the steel, which affects the phase fraction of the final microstructure along with the cooling rate, and therefore shows a great correlation with the tensile strength. Next, alloy components may affect the final quality of steel by affecting the mechanical properties of the martensite or bainite phase itself included in the final microstructure. In this respect, the contribution to increase the strength of martensite is large and the content of carbon (C), an element that increases the hardenability, is often high. These are alloying elements that are mainly added for securing. However, the more alloying elements are added to secure the hardenability and thus strength of steel, the more often the impact toughness and ductility, which are in inverse proportion to the strength, deteriorate. It should be.

Japanese Patent Publication No. JP2011-001620 (2011.01.06) Korean Patent Publication No. KR2016-0063532 (2016.06.07)

An object of the present invention is to provide a high-strength steel material with excellent physical properties, particularly impact toughness, and a manufacturing method thereof, compared to steel materials used in existing plants, pressure vessels, storage tanks, and the like.

The object of the present invention is not limited to the above. The subject of the present invention will be understood from the entire contents of this specification, and those skilled in the art will have no difficulty in understanding the additional subject of the present invention.

One aspect of the present invention,

By weight%, carbon (C): 0.04 to 0.08%, manganese (Mn): 1.0 to 2.0%, silicon 0.01 to 0.5%, phosphorus (P): 0.02% or less, sulfur (S): 0.01% or less, chromium ( Cr): 1.0 to 2.0%, Molybdenum (Mo): 0.2 to 0.8%, Aluminum (Al): 0.005 to 0.5%, Titanium (Ti) 0.01 to 0.03%, Nitrogen (N): 0.001 to 0.006, balance Fe and others contains unavoidable impurities;

It relates to a high-strength steel having excellent low-temperature impact toughness and having a steel microstructure including 70 to 90% of self-tempered martensite and the balance self-tempered bainite in area %.

The steel material may have a tensile strength of 485 MPa or more and an impact absorption energy of 100 J or more at -40 ° C.

Another aspect of the present invention is

Heating the steel slab having the above-described alloy components in a temperature range of 1050 ~ 1250 ℃;

Finishing hot-rolling the heated steel slab at 900° C. or higher to prepare a hot-rolled steel sheet, followed by air-cooling to room temperature;

reheating the cooled hot-rolled steel sheet to a temperature range of 850 to 950° C.;

water-cooling the reheated hot-rolled steel sheet to a temperature range of 300 to 400° C. at a cooling rate of 10 to 60° C./s; and

It provides a method for producing a high-strength steel having excellent impact toughness, including the step of air-cooling the water-cooled hot-rolled steel sheet after self-tempering heat treatment in a temperature range of 450 to 550 ° C.

After the reheating, it is preferable to hold for 20 minutes or more.

The self-tempering heat treatment may be performed in the range of 30 to 300 minutes from the highest recuperation temperature to room temperature.

According to the present invention having the above configuration, it is possible to provide a high-strength steel having excellent impact toughness. Therefore, the steel material of the present invention has an effect that can be usefully applied to plants, pressure vessels, storage tanks, etc. used in various environments including cold regions.

1 is a schematic diagram showing a heat treatment process in which self-tempering is performed after quenching according to an embodiment of the present invention.

Hereinafter, the present invention will be described.

In order to secure high strength of steel materials used in existing plants, pressure vessels, storage tanks, etc., the content of hardenability enhancing alloy elements including carbon was increased and quenching-tempering was performed. However, in the case of these steels, when used in an environment including cold regions, low-temperature impact toughness was rapidly reduced, making it impossible to apply them. In order to solve this problem, if the concentration of the alloy is adjusted, such as reducing the carbon content, there is a problem that the strength is reduced and the required mechanical properties of the steel are not satisfied. The problem of increasing time and cost still existed.

Accordingly, the present inventors conducted extensive research to develop a steel having excellent low-temperature impact toughness while having strength suitable for use in plants, pressure vessels, and storage tanks for cold regions. As a result, it was confirmed that a steel having a tensile strength of 485 MPa or more and an impact toughness of 100 J or more at -40 ° C can be manufactured when a microstructure advantageous to securing the intended physical properties is formed while optimizing the alloy composition and manufacturing conditions. did In particular, it was found that steel having the above-described characteristics can be manufactured even with one heat treatment process of self-tempering after quenching after two heat treatment processes of quenching and tempering, and accordingly, the time and cost required for production It is also to confirm that the effect of reducing can be achieved and to suggest the present invention.

The high-strength steel with excellent low-temperature impact toughness of the present invention, in weight percent, contains carbon (C): 0.04-0.08%, manganese (Mn): 1.0-2.0%, silicon 0.01-0.5%, phosphorus (P): 0.02% Below, sulfur (S): 0.01% or less, chromium (Cr): 1.0 ~ 2.0%, molybdenum (Mo): 0.2 ~ 0.8%, aluminum (Al): 0.005 ~ 0.5%, titanium (Ti) 0.01 ~ 0.03%, Nitrogen (N): 0.001 to 0.006, the balance including Fe and other unavoidable impurities, in area%, a steel microstructure containing 70 to 90% self-tempered martensite and the balance self-tempered bainite have

Hereinafter, the reason for limiting the alloy composition of the steel sheet provided in the present invention as described above will be described in detail.

Meanwhile, in the present invention, unless otherwise specified, the content of each element is based on weight, and the ratio of tissue is based on area.

·Carbon (C): 0.04~0.08%

Carbon (C) is an element that has the greatest influence on the mechanical properties of steel, and its content needs to be appropriately controlled. Although carbon atoms are interstitially distributed between iron atoms and contribute to strength, in the manufacturing method including water cooling as in the steel of the present invention, it has a great effect on the hardenability of the steel, resulting in the area fraction of the final microstructure and It is an element that greatly affects the resulting mechanical properties. If the content of C is less than 0.04%, it is difficult to use it as a material for the intended use in the present invention due to a decrease in martensite fraction and weakening of matrix strength due to lack of hardenability. On the other hand, when the content exceeds 0.08%, the strength is excessively increased, making processing difficult, and in particular, there is a problem in that the low-temperature toughness to be implemented in the present invention is lowered.

Therefore, the C is preferably limited to 0.04 to 0.08%, more preferably to control the content in the range of 0.06 to 0.08%.

Manganese (Mn): 1.0~2.0%

Manganese (Mn) is an element that is advantageous for securing the strength of a steel sheet by increasing hardenability of steel and is an element that is advantageous compared to other alloys in terms of economy, so it is the most commonly used alloying element for steel materials. In particular, since the carbon content of the steel of the present invention is significantly lower than that of conventional quenching-tempering steels in order to secure low-temperature impact toughness, the content of Mn, an alternative hardenability element, affects the final mechanical properties of the steel. The impact is great. If the Mn content is less than 1.0%, it is impossible to secure the target strength due to lack of hardenability, and if the content exceeds 2.0%, a segregation zone such as MnS is severely generated inside the steel and acts as the starting point of fracture , rather, the low-temperature impact toughness may be inferior.

Therefore, in the present invention, the Mn is preferably included in the range of 1.0 to 2.0%, more preferably, the Mn is included in the range of 1.2 to 1.8%.

·Silicon (Si) : 0.01~0.5%

Silicon (Si) is used as a deoxidizer and is useful because it has an effect of improving strength, but when added excessively, low-temperature impact toughness is reduced and weldability is also deteriorated at the same time. In addition, there is a problem that the Si compound is liquefied in the surface layer of the steel material in the heating furnace and there is a possibility of destabilizing the quality of the surface such as scale.

Therefore, in the present invention, it is preferable to include the Si in the range of 0.01 to 0.5%, more preferably, the Si in the range of 0.2 to 0.4%.

Phosphorus (P): 0.02% or less

Phosphorus (P) is an element that is advantageous for improving the strength of steel and securing corrosion resistance, but since it is an element that greatly inhibits impact toughness, it is advantageous to control it as low as possible.

In the present invention, even if the P is contained in a maximum of 0.02%, there is no great difficulty in securing the intended physical properties, and the P content may be limited to 0.02% or less. However, 0% can be excluded considering the level that is unavoidably added.

Sulfur (S): 0.01% or less

Sulfur (S) is an element that greatly inhibits the impact toughness of steel by combining with Mn in steel to form non-metallic inclusions such as MnS. Accordingly, it is advantageous to control the S as low as possible.

In the present invention, even if the S is contained at a maximum of 0.01%, there is no great difficulty in securing the intended physical properties, and the content of S may be limited to 0.01% or less. However, 0% can be excluded considering the level that is unavoidably added.

Chromium (Cr): 1.0 to 2.0%

Chromium (Cr) is an element that has a great effect on strength by increasing hardenability. In particular, in the present invention, the addition of 1.0% or more is preferable because the alloy of carbon is limited and the hardenability required for steel is secured with manganese, molybdenum, and chromium. However, when added too excessively, a problem of significantly deteriorating weldability may occur.

Therefore, in the present invention, it is preferable to limit the chromium content to 1.0 to 2.0%, more preferably to limit the chromium content to 1.0 to 1.5%.

Molybdenum (Mo): 0.2 to 0.8%

Molybdenum (Mo) increases the hardenability of steel, suppresses the production of ferrite at high temperatures, and increases strength by generating low-temperature phases such as low-temperature bainite or martensite. In addition, the reduction in ductility and low-temperature impact toughness caused by the addition of alloys is also an element that is at a lower level than other alloys. In addition, since it is an alloying element with a fairly large precipitation strengthening effect, it is an element that can compensate for the deterioration of various mechanical properties caused by the low addition of C to be practiced in the present invention. However, it is necessary to limit the upper limit because it can excessively increase the hardness of the welded part, and since it is an expensive alloy element, it can reduce the economic feasibility of commercial steel.

Therefore, in consideration of this, in the present invention, the Mo content is preferably limited to 0.2 to 0.8%, more preferably, to be limited to 0.3 to 0.6%.

·Aluminum (Al): 0.005 to 0.5%

Aluminum (Al) is an element effective for inexpensively deoxidizing molten steel, and may contain 0.005% or more for this purpose. However, when the content exceeds 0.5%, there is a problem of causing nozzle clogging during continuous casting, and the dissolved Al forms an island martensite phase at the welded portion, which may reduce the toughness of the welded portion.

・Titanium (Ti): 0.01 to 0.03%

Titanium (Ti) is combined with nitrogen (N) in steel to form fine nitrides to relieve grain coarsening that may occur near the welding fusion line, thereby suppressing the decrease in toughness. If the content of Ti is excessively low, the number of Ti nitrides is insufficient and the effect of suppressing grain coarsening is insufficient. However, since there is a problem that coarse Ti nitride is generated when added too excessively and the grain boundary fixing effect is lowered, considering this, it can be limited to 0.03% or less.

Nitrogen (N): 0.01% or less

Nitrogen (N) combines with Ti in steel to form fine nitrides, and suppresses deterioration in toughness by alleviating grain coarsening that may occur near the welding fusion line. However, if the content is excessive, the toughness is greatly reduced, so in consideration of this, it can be limited to 0.01% or less, 0% is excluded.

The remaining component of the present invention is iron (Fe). However, since unintended impurities from raw materials or the surrounding environment may inevitably be mixed in a normal manufacturing process, this cannot be excluded. Since these impurities are known to anyone skilled in the ordinary manufacturing process, not all of them are specifically mentioned in this specification.

The steel material of the present invention having the above-described alloy components may include a martensite phase and a bainite phase as a microstructure. At this time, the martensite phase and the bainite phase have self-tempered martensite and self-tempered bainite structures in a state tempered by self-tempering.

Specifically, the self-tempered martensite phase may include 70 to 90% by area%, and it is preferable to include the self-tempered bainite phase as the remaining structure. At this time, the bainite phase means a low-temperature bainite phase, and although a very small amount (<0.5wt%) of retained austenite may be observed between packets, which are internal structures in martensite, it is regarded as part of the martensite structure.

If the fraction of the self-tempered martensite phase is less than 70%, it is not possible to sufficiently secure the tensile strength of the steel, whereas if the fraction exceeds 90%, the fraction of the relatively soft bainite phase is lowered, resulting in a target level of low-temperature impact toughness will not be able to obtain

The steel material of the present invention having the above-described steel composition and steel microstructure can secure an impact toughness of 100J or more at -40 ° C together with a tensile strength of 485 MPa or more.

Hereinafter, a method for manufacturing a high-strength steel having excellent low-temperature impact toughness, which is another aspect of the present invention, will be described in detail.

The steel of the present invention can manufacture a steel slab satisfying the alloy components proposed in the present invention through the process of [heating - hot rolling - cooling - reheating - water cooling]. - It is possible to secure the final intended microstructure by tempering (self-tempering).

Hereinafter, each process condition will be described in detail.

[Heating of steel slabs]

In the present invention, the steel slab may be heated prior to hot rolling to dissolve the Ti or Mn compound formed during casting, and at this time, the heating process may be performed in a temperature range of 1050 to 1250 ° C.

If the heating temperature of the steel slab is less than 1050 ° C., the compound cannot be sufficiently re-dissolved, and there is a problem that the coarse compound remains. On the other hand, when the temperature exceeds 1250 ° C., it is not preferable because the strength is lowered due to abnormal grain growth of austenite grains.

[Hot rolling]

The reheated steel slab may be hot-rolled to produce a hot-rolled steel sheet, and at this time, after rough rolling under normal conditions, finish hot-rolling may be performed at a constant temperature.

In the case of the present invention, since reheating is performed on the hot-rolled hot-rolled steel sheet, the temperature during the finish hot-rolling is not particularly limited. However, if the temperature is too low, the load of hot rolling increases and the shape of the steel strip tends to deteriorate.

[Cooling and reheating]

After air-cooling the hot-rolled steel sheet manufactured as described above to room temperature, it may be reheated to a temperature at which a certain fraction of austenite is generated for quenching heat treatment.

Since the particle size increases and the hardenability increases as the reheating temperature increases, the higher the reheating temperature is, the more advantageous it is to secure strength. However, if the temperature is too high, the grain size of austenite becomes excessively coarse, resulting in poor low-temperature impact toughness. Therefore, in the present invention, the reheating may be performed in a temperature range of 850 to 950 ° C.

After reheating the hot-rolled steel sheet to the above-mentioned temperature, it can be maintained so that heat can be sufficiently transferred to the inside of the steel. The holding time at this time is not particularly limited, but it is 20 so that austenite phase transformation and grain growth can occur sufficiently It can take more than a minute.

[Water cooling and self-tempering heat treatment]

After sufficiently transferring heat to the inside of the hot-rolled steel sheet by reheating according to the above, it is quenched by water cooling, and then self-tempering heat treatment may be performed.

The water cooling may be performed at a cooling rate of 10 to 60° C./s, and may be terminated in a temperature range of 300 to 400° C. for the self-tempering heat treatment, which is a subsequent process.

If the cooling end temperature is less than 300 ° C, the heat in the hot-rolled steel sheet becomes insufficient and the subsequent self-tempering heat treatment cannot be performed properly, whereas if the temperature exceeds 400 ° C, the area fraction of the bainite phase generated during cooling becomes excessive. and the martensite phase may become insufficient in the final structure.

In addition, if the cooling rate during the water cooling is less than 10 ° C / s, the tensile strength of the steel sheet may be lowered due to insufficient martensite phase fraction, on the other hand, if it exceeds 60 ° C / s, the full martensitic structure is obtained, thereby increasing the impact toughness of the steel sheet. this may deteriorate.

The hot-rolled steel sheet water-cooled in the above-mentioned temperature range is reheated and the temperature is increased, and as shown in FIG. 1, self-tempering heat treatment may be performed in the temperature range of 550° C.

1 is a schematic diagram showing a heat treatment process in which self-tempering is performed after quenching according to an embodiment of the present invention.

During the self-tempering heat treatment, the martensite structure of a certain area fraction generated during the quenching of the surface layer of the steel material is tempered, and the internal stress is relieved, through which a slight decrease in strength and an improvement in impact toughness are achieved. In addition, lower bainite transformation occurs in the residual austenite structure. At this time, bainite transformation exotherm occurs, so that the recuperation temperature measured outside the steel sheet is partially increased.

On the other hand, during the self-tempering heat treatment, the center of the steel material stops cooling at a higher temperature than the surface layer, so it has a relatively low martensite fraction, and the temperature does not rise immediately after cooling stops, but the lower bainite transformation starts after a certain period of time In this case, the martensitic structure previously generated by the transformation heating is tempered and the impact toughness is improved.

In the case of self-tempering heat treatment, the maximum temperature of recuperation is determined by the cooling stop temperature and the lower bainite fraction to be transformed, and the control of the recuperation temperature is not possible, but the temperature is excessively recuperated so that the self-tempering heat treatment recovery temperature exceeds 550 ° C. When the tempering of martensite is excessive, the target strength cannot be secured, and if the recuperation temperature is lower than 450° C., the relaxation of internal stress is insufficient, making it impossible to secure impact toughness.

The time of the self-tempering heat treatment is not particularly limited, but may be performed in the range of 30 minutes to 300 minutes from the highest recuperation temperature to room temperature.

Thereafter, after completing the self-tempering heat treatment according to the above, the final steel material may be obtained by air-cooling to room temperature.

Hereinafter, the present invention will be described in more detail through examples. It should be noted that the following examples are only for understanding of the present invention, and are not intended to specify the scope of the present invention.

(Example)

After preparing a steel slab having alloy components shown in Table 1 below, hot-rolled steel sheets were manufactured by performing each process according to the manufacturing conditions shown in Table 2 below.

After taking a tensile specimen in the width direction for each hot-rolled steel sheet manufactured, the microstructure was observed, and tensile strength at room temperature (approximately 25 ° C) and impact toughness at -40 ° C were measured. At this time, the microstructure was observed at × 200 magnification using an optical microscope, and then the area fraction of each phase was measured by applying the Banding Index measurement method based on the ASTM E 1268 standard. In addition, tensile strength was measured according to JIS No. 5 standard, and low-temperature impact toughness was measured using a Charpy impact tester according to ASTM E 23. Each resultant value is shown in Table 3 below.

steel grade Alloy composition (% by weight) C Mn Si P S Cr Mo Al Ti N Invention Steel 1 0.06 1.52 0.06 0.007 0.001 1.98 0.73 0.19 0.02 0.003 Invention Steel 2 0.07 1.65 0.30 0.006 0.001 1.44 0.69 0.34 0.01 0.004 Invention Steel 3 0.08 1.01 0.27 0.010 0.001 1.37 0.28 0.05 0.02 0.004 Comparative Lecture 1 0.12 1.45 0.34 0.008 0.001 1.35 0.25 0.03 0.01 0.006 comparative steel 2 0.03 1.9 0.45 0.01 0.002 1.86 0.72 0.05 0.017 0.006 comparative lecture 3 0.07 0.84 0.37 0.01 0.001 1.12 0.77 0.03 0.015 0.004 comparative lecture 4 0.08 1.45 0.47 0.008 0.002 0.05 0.12 0.01 0.012 0.008

steel grade manufacturing conditions slab heating temperature
(℃) finish rolling end temperature
(℃) heat treatment reheat temperature
(℃) cooling rate
(℃/s) cooling stop temperature
(℃) maximum recuperation temperature
(℃) division invention steel 1 1-1 1139 935 890 45 351 499 Invention example 1 1-2 1020 901 900 45 377 487 Comparative Example 1 1-3 1120 910 910 9 398 502 Comparative Example 2 invention steel 2 2-1 1090 903 910 16 366 540 Invention example 2 2-2 1077 920 870 80 310 460 Comparative Example 3 2-3 1210 911 855 50 190 460 Comparative Example 4 invention steel 3 3-1 1210 998 910 60 358 422 Invention Example 3 3-2 1140 915 880 55 310 420 Comparative Example 5 3-3 1160 942 890 45 385 565 Comparative Example 6 comparative steel 1 1-1 1228 937 926 21 337 533 Comparative Example 7 1-2 1208 977 899 24 370 550 Comparative Example 8 comparative steel 2 2-1 1181 936 929 23 334 460 Comparative Example 9 2-2 1113 903 885 24 375 450 Comparative Example 10 comparative lecture 3 3-1 1083 919 914 44 354 459 Comparative Example 11 3-2 1184 943 931 31 317 505 Comparative Example 12 comparative lecture 4 4-1 1150 973 919 12 307 469 Comparative Example 13 4-2 1128 928 925 28 393 519 Comparative Example 14

division Microstructure (area fraction%) mechanical properties self tempered martensite Self-Tempered Bainite tensile strength
(MPa) impact toughness
(@-40℃, J) Invention example 1 82 18 537 209 Comparative Example 1 65 35 468 337 Comparative Example 2 55 45 432 360 Invention example 2 72 28 499 166 Comparative Example 3 100 0 589 17 Comparative Example 4 95 5 577 48 Invention example 3 88 12 555 137 Comparative Example 5 88 12 527 90 Comparative Example 6 79 21 480 216 Comparative Example 7 92 8 580 27 Comparative Example 8 88 12 575 43 Comparative Example 9 72 28 437 428 Comparative Example 10 76 24 446 399 Comparative Example 11 65 35 425 289 Comparative Example 12 60 40 417 405 Comparative Example 13 67 33 448 378 Comparative Example 14 69 31 465 399

As shown in Tables 1 to 3, Inventive Examples 1 to 3 satisfying all of the alloy components and manufacturing conditions proposed in the present invention have a tensile strength of 485 MPa or more and an impact toughness of 100 J or more at -40 ° C. It can be confirmed that the toughness is excellently secured.

On the other hand, in Comparative Example 1, the slab heating temperature was applied significantly lower than the manufacturing conditions of the present invention, and the added alloy components were not sufficiently dissolved in the slab, so that the hardenability of the steel was insufficient and a sufficient fraction of martensite was not generated. showed a low level of intensity.

Also, in Comparative Example 2, when the cooling rate during quenching was lower than the manufacturing conditions of the present invention, a large amount of bainite was formed during cooling and a relatively small fraction of martensite was formed, indicating a low level of tensile strength.

On the other hand, in Comparative Example 3, when the cooling rate during quenching is higher than the manufacturing conditions of the present invention, the entire structure is phase transformed to martensite, so that high tensile strength can be secured, but the impact toughness is poor.

Comparative Example 4 is a case where the cooling stop temperature after quenching is significantly lower than 300 ° C., which is the manufacturing condition of the present invention, and, like Comparative Example 3, an excessive amount of martensite is generated than intended, resulting in poor impact toughness. showed

Comparative Example 5 is a case where the self-tempering temperature is controlled lower than the manufacturing conditions of the present invention because recuperation does not occur sufficiently after cooling, and an appropriate amount of phase fraction is secured during cooling, but stress relaxation of the generated martensite and bainite is sufficient. As a result, the impact toughness was poor.

Comparative Example 6 is a case where excessive recuperation occurred after cooling, and, contrary to Comparative Example 5, excessive stress relaxation was applied to the martensite and bainite produced, resulting in poor tensile strength.

On the other hand, Comparative Examples 7 to 14 are comparative examples in which the manufacturing conditions of the present invention were applied, but the composition was outside the scope of the present invention and the target strength or impact toughness was not secured.

In Comparative Examples 7 to 8, the carbon content of Comparative Steel 1 was excessive, and an excessive amount of martensite was generated due to high hardenability, respectively, or the matrix strength of the resulting structure was excessively high, resulting in relatively poor impact toughness. was

On the other hand, in Comparative Examples 9 to 10, the carbon content of Comparative Steel 2 was too low and the matrix strength of the resulting microstructure was too low, and the tensile strength was inferior even though a sufficient amount of martensite was secured.

In Comparative Examples 11 to 14, the content of manganese, chromium, and molybdenum in Comparative Steels 3 to 4 was not sufficient, resulting in poor hardenability of the steel, so that a sufficient amount of martensite was not produced, so the impact toughness was excellent, but the tensile strength was too hot. performance for the

As described above, the detailed description of the present invention has been described with respect to the preferred embodiments of the present invention, but those skilled in the art to which the present invention belongs can make various modifications without departing from the scope of the present invention. Of course this is possible. Therefore, the scope of the present invention should not be limited to the described embodiments and should not be defined, and should be defined by not only the claims described later, but also those equivalent thereto.

Claims (5)

By weight%, carbon (C): 0.04 to 0.08%, manganese (Mn): 1.0 to 2.0%, silicon 0.01 to 0.5%, phosphorus (P): 0.02% or less, sulfur (S): 0.01% or less, chromium ( Cr): 1.0 to 2.0%, Molybdenum (Mo): 0.2 to 0.8%, Aluminum (Al): 0.005 to 0.5%, Titanium (Ti) 0.01 to 0.03%, Nitrogen (N): 0.001 to 0.006, balance Fe and others contains unavoidable impurities;
Has a steel microstructure, by area%, of 70 to 90% self-tempered martensite and the balance self-tempered bainite, and
High-strength steel for pressure vessels with excellent low-temperature impact toughness with a tensile strength of 485 MPa or more and shock absorption energy of 100 J or more at -40 ° C.
delete By weight%, carbon (C): 0.04 to 0.08%, manganese (Mn): 1.0 to 2.0%, silicon 0.01 to 0.5%, phosphorus (P): 0.02% or less, sulfur (S): 0.01% or less, chromium ( Cr): 1.0 to 2.0%, Molybdenum (Mo): 0.2 to 0.8%, Aluminum (Al): 0.005 to 0.5%, Titanium (Ti) 0.01 to 0.03%, Nitrogen (N): 0.001 to 0.006, balance Fe and others Heating a steel slab containing unavoidable impurities in a temperature range of 1050 to 1250 ° C;
Finishing hot-rolling the heated steel slab at 900° C. or higher to prepare a hot-rolled steel sheet, followed by air-cooling to room temperature;
reheating the cooled hot-rolled steel sheet to a temperature range of 850 to 950° C.;
water-cooling the reheated hot-rolled steel sheet to a temperature range of 300 to 400° C. at a cooling rate of 10 to 60° C./s; and
Including; air-cooling after self-tempering heat treatment of the water-cooled hot-rolled steel sheet in a temperature range of 450 to 550 ° C.
The self-tempering heat treatment is a method for producing a high-strength steel for pressure vessels according to claim 1, which has excellent impact toughness and is performed in the range of 30 to 300 minutes from the highest recuperation temperature to room temperature.
[Claim 4] The method of manufacturing a high-strength steel for pressure vessels having excellent impact toughness according to claim 3, wherein the reheating is maintained for 20 minutes or more.
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